The present invention relates to an electrically compensated electrohydraulic servo system with position related feedback loop in combination with a servovalve having a differential pressure transducer connected across the lines or conduits that communicate with the drive means. One aspect of the present invention is the combination of bridged-T and lead-lag electrical networks with a servovalve having a differential pressure transducer feedback connected across the lines to the motor and a feedback loop to provide a signal representative of motor movement. An aspect is the provision of a combination as in the preceding sentence to drive a motor such as a hydraulic cylinder to linearly position a slide element in a machine in connection with which the invention without intention of being limited thereto will be described.
Electrohydraulic servosystems are frequently troubled by vibrations. Systems with feedback loops especially may become unstable when the vibration frequency approaches or equals a resonant frequency of the system because of the additive effects sometimes resulting from the feedback. Where exact positioning, smooth operation, acoustical and control noise reduction, and holding (i.e. position holding) are important factors, as in machine tool positioning, reduction of vibration problems is important if the machine tool is to perform its function with the degree of precision required and if products thereof are to have correct dimensions and surface finish.
The present invention provides a low damped, low gain system with enhanced vibration dampening and ability to block out the primary resonant frequency whereby a system of ostensible high gain and high damping is achieved.
The present invention comprehends an electrically compensated electrohydraulic position control servo system having a negative feedback loop for controllably positioning a drive means said system comprising a bridged-T network; a source of a reference signal; a summing junction connected between said source and said bridged-T to provide an error signal as an input signal to said bridged-T; a dynamic pressure feedback servovalve assembly comprising a servovalve having as an output a pair of hydraulic lines for connection to said drive means as supply and return lines thereof, a differential pressure transducer connected as a feedback loop between said lines in pressure communication with the hydraulic flow area within each such line, and electric power means to position said servovalve; a lead-lag network connected between said bridged-T and said power means; a position transducer connected in said position feedback loop between the output from said drive means and said summing junction; and means for delivering a control signal from the output of said lead-lag network to said servovalve assembly; whereby position reference signals from said source position said drive means in a lowly damped low gain system of enhanced stiffness and minimized overshoot.
Certain components and subcombinations used in this invention are admitted to be per se old: Bridged-T and lead-lag networks both alone and in combination are per se old (See Merritt infra pp. 246-258 and See AIEE -- now named IEEE -- Paper No. 59-220, July 1959, Chandaket and Rosenstein "Notes on Bridged-T . . . Network Loading" pp. 148, 162). Dynamic or differential pressure feedback servovalves per se are also admitted to be old (Merritt "Hydraulic Control Systems" -- publ. 1967 John Wiley, Catalog Card No. 66 - 28759 -- p. 140, 141; Blackburn et al. "Fluid Power Control" -- publ. 1960 The M.I.T. Press Card No. 59-6759 -- pp. 518).